17 foot Casita rear wondow leak
Agree with Gordon. Who did the installation? Point of clarification: was the whole window replaced, or just the broken glass?
Even if I were having someone else fix the leak, it never hurts to do a little testing yourself. A container with a pour spout will allow for more precise aiming of water to pin down the leak. Work from the bottom up.
There are three common ways a window can leak (from most to least likely): (1) It can leak around the outer flange where it is sealed to the fiberglass shell, caused by a small gap in the butyl sealant. That would be the first thing to check on a new installation. (2) It can leak at the bottom channels where the window panes slide. If the drain slots (weep holes) are blocked with debris or incompletely punched on a new window (a "hanging chad"), rain fills the channels and overflows into the trailer. (3) it can leak around the internal seals between the glass and the frame. That's more common on an older window, but if a pane was replaced, it might not be completely sealed into the frame.